The development of apple (Malus×domestica) cultivars carrying natural resistance against major pathogens and pests is a crucial component of any successful breeding strategy. The two most significant fungal diseases of apple are apple scab and powdery mildew. Powdery mildew (caused by Podosphaera leucotricha) is a particularly serious problem in relatively dry apple growing areas but is prevalent in all growing regions.
A number of sources of resistance to powdery mildew have been identified in non-commercial apples and breeding to incorporate these resistances into new commercial varieties is in progress using various strategies [11, 13, 26, 27].
Two sources of resistance in particular have been used in a number of different breeding programmes. These sources of powdery mildew resistance are: 1. An open-pollinated seedling of Malus zumi (MAL68/5) carrying the Pl2 resistance locus, and 2. An open-pollinated seedling of Malus robusta (MAL59/9) carrying the Pl1 resistance locus. Genetic analysis of these sources has indicated that in some genetic backgrounds at least these loci appear to segregate as a single major dominant locus for resistance [10, 24, 29]. Other powdery mildew resistance loci that have been genetically characterized include PlMIS [8], Pld [40], Pl8 [25] and Plw [15]. None of the genes responsible for these resistances have been isolated.
To date, about 70 resistance genes have been cloned from at least 14 different plant species conferring resistance to various diseases [28]. The encoded proteins have been grouped into classes based on a number of characteristic domains.
The first class consists of genes encoding proteins with characteristics of serine/threonine (S/T) kinases. This class includes the first cloned plant disease resistance gene, Pto from tomato [29]. This class also includes two close relatives of the Pto gene, the LhirPto [33] and Fen [30, 34] genes, and the Rpg1 gene [4].
The second class of resistance genes consists of those encoding proteins containing a central nucleotide binding site (NBS) and a carboxy terminal leucine-rich repeat (LRR). The first of these genes to be cloned were the Arabidopsis thaliana RPS2 [2] and Nicotiana tabacum N [41] genes. The N gene represents the first member of a subclass with Toll-Interleukin-1 like receptor domains at the amino terminus. The RPS2 gene represents the first member of the CC-NBS-LLR subclass with leucine zippers or coiled coil (CC) motifs at the amino terminus. This subclass is sometimes also referred to as non-TIR.
A third major class of resistance genes, the xLRR class, consists of those encoding proteins composed almost entirely of leucine rich repeats (LRRs) that are predicted to reside in an extracellular environment based on their amino acid sequence [21]. The Cf-9 gene [22] was the first gene cloned in this class. Most of the genes in this class have been cloned from tomato (Cf genes) and confer resistance against the leaf mold Cladosporium fulvum. The Vf gene from apple confers resistance to apple scab and belongs to the xLRR class of resistance genes [1].
A fourth class of resistance genes consists of those encoding proteins with an amino terminal serine/threonine protein kinase domain with homology to the Pto gene, a carboxy terminal LRR domain with homology to the Cf genes and a central putative transmembrane region [38]. These genes have all the hallmarks of a transmembrane receptor kinase. Receptor kinases are often involved in mammalian ligand mediated signalling (e.g. hormone receptors) with the protein kinase acting as the signalling domain inside the cell and the LRR conferring specificity in the extracellular environment [3]. The Xa21 gene from rice is a member of this class.
A small number of other disease resistance genes that do not fit neatly into one of these four classes have recently been cloned. These include the mlo gene [5] from barley, the Hs1Pro-1 gene [6] from sugar beet and the Ve genes [23] from tomato. The mlo gene has a putative 7 transmembrane structure and shares no domains with other cloned resistance genes whereas the Hs1Pro-1 gene contains LRRs and the Ve gene contains LRRs, PEST sequences, leucine zippers and potential signals for receptor mediated endocytosis.
Powdery mildew resistance in apple is subject to heterogeneity at the phenotypic, and possibly also genetic, levels. Typically resistant progeny are not reliably identifiable based on nursery phenotypes [20] or using (macroscopic) symptom development in the field. Because of this, resistance is sometimes not scored until the plants have matured in the orchard over several years [10]. This makes the screening for resistance against this important pathogen of apple by traditional means especially difficult and time consuming.
The cloning of a gene for resistance against apple powdery mildew would constitute a significant advance and would have a number of advantages over the traditional breeding routes for resistance.
It is therefore an object of the invention to provide compositions and methods useful for conferring powdery mildew resistance in plants and/or at least to provide the public with a useful choice to this end.